What is heat treatment？
Heat treatment refers to a metal thermal processing process in which materials are heated, kept warm and cooled in a solid state to obtain the desired structure and properties. During the progression from the Stone Age to the Bronze and Iron Ages, the role of heat treatment was gradually recognized.
As early as 770 BC to 222 BC, the Chinese had discovered in production practice that the properties of steel would change due to the influence of temperature and pressure deformation. The softening treatment of white cast iron is an important process for making agricultural tools.
Heat treatment includes normalizing, annealing, quenching, tempering, carburizing, conditioning, etc.
1. Normalizing: heating steel or steel parts to an appropriate temperature above the critical point AC3 or ACM and keeping it for a certain period of time, then cooling in the air to obtain a pearlite-like structure.
2. Annealing annealing: heating the hypoeutectoid steel workpiece to 20-40 degrees above AC3, holding it for a period of time, then cooling slowly with the furnace (or buried in sand or lime) to below 500 degrees and cooling in the air.
3. Quenching: A heat treatment process in which the steel is austenitized and then cooled at an appropriate cooling rate to cause the workpiece to undergo martensite and other unstable structural transformations in the entire cross-section or within a certain range.
4. Tempering: Heat the quenched workpiece to an appropriate temperature below the critical point AC1 for a certain period of time, and then cool it with a method that meets the requirements to obtain the required structure and properties.
5. Carbonitriding of steel: Carbonitriding is the process of simultaneously infiltrating carbon and nitrogen into the surface of steel. Customarily, carbonitriding is also called cyanidation, and medium-temperature gas carbonitriding and low-temperature gas carbonitriding (that is, gas soft nitriding) are widely used. The main purpose of medium temperature gas carbonitriding is to improve the hardness, wear resistance and fatigue strength of steel. Low-temperature gas carbonitriding is mainly nitriding, and its main purpose is to improve the wear resistance and seizure resistance of steel.
6. Quenching and tempering: Generally, the heat treatment combined with quenching and high temperature tempering is called quenching and tempering. Quenching and tempering treatment is widely used in various important structural parts, especially those connecting rods, bolts, gears and shafts that work under alternating loads. After quenching and tempering treatment, the tempered sorbite structure is obtained, and its mechanical properties are better than the normalized sorbite structure with the same hardness. Its hardness depends on the high temperature tempering temperature and is related to the tempering stability of the steel and the cross-sectional size of the workpiece, generally between HB200-350.
